Rickettsia rickettsii is the causative agent of Rocky Mountain spotted fever. Current incidence averages approximately 1000 individuals per year in the United States. Related members of the spotted fever group include the rickettsial agents of Mediterranean spotted fever, Australian tick typhus, North Asian tick typhus, and rickettsialpox as well as several non-pathogenic species. Other members of the genus Rickettsia are the etiologic agents of endemic and epidemic typhus. Efforts have been focused on a pair of immunodominant surface protein antigens with estimated mol. masses of 190 and 135 kDa and termed rOmp A and B, respectively. These proteins are of interest due to their surface location, strong immunogenicity, and reactivity with monoclonal antibodies that protect mice against lethal rickettsial challenge. Our current objectives are to determine the role of these proteins in rickettsial interaction with the host cell. These studies involve characterization of the variability in both protein and gene structure between species of rickettsiae that differ in virulence both in animal models and cell culture. In the past year we have shown that the rOmp B protein is encoded by an unusually large open reading frame and then processed to yield stable membrane associated products of 135 and 32 kDa. A spontaneously occurring mutant that is unable to complete this processing step is avirulent. The rOmp A protein, a protective immunogen, has 13 direct repeat units of 72 - 75 amino acids each that, although similar, can be categorized as type I or II repeats. DNA sequence analysis of the rOmp A protein from diverse species of rickettsiae indicates variation in the order of the repeat units but little sequence divergence between corresponding type I or II units. In addition, we are initiating studies to determine the host cells response to infection by intracellular parasites. Host cell responses so far detected include changes in actin structure possibly mediated by the parasite and aspects of signal transduction such as phospholipase C activity, inositol phosphate production, and protein kinase C activity.